The present invention relates to a protective circuit for a fluorescent lamp with a first and a second lamp filament, comprising a DC voltage source with a positive and a negative pole, a half-bridge arrangement with a first and a second switch, the half-bridge arrangement being fed by the DC voltage source, and the first and the second switch being interconnected to form a first reference point, the first reference point being connected to the negative pole via a first resistor, a decoupling capacitor that is arranged in a serial connection between the half-bridge arrangement and the first or the second lamp filament, the connection of the decoupling capacitor on the filament side forming a second reference point that is connected to the negative pole via a second resistor, a comparator that has a first and a second input and an output, the first input being connected to the first reference point, and the second input being connected to the second reference point, the output being connected to the negative pole via a detection capacitor and an evaluation circuit with the aid of which the voltage dropping across the detection capacitor can be evaluated in order to deactivate the half-bridge arrangement upon overshooting of a predetermined voltage level.
Such a protective circuit is known and is, for example, installed by the Applicant of the present invention in ballasts for fluorescent lamps. The protective circuit consists in evaluating at the end of the service life of the fluorescent lamp, that is to say when the lamp is not yet defective, a criterion that leads in good time before overheating in the filament region (risk of fusing of the base) to a shutdown of the half-bridge arrangement (also known as end-of-life shutdown). Use is made in this case of the fact that the filaments of a fluorescent lamp are covered with emitter in order to reduce the work function of the electrons. In the closure phase, the absence of the emitter on one of the two lamp filaments of the fluorescent lamp becomes noticeable by virtue of the fact that the work function slowly increases again, and the voltage dropping across the decoupling capacitor changes thereby. In normal operation, that is to say when both filaments still have emitter, the two reference points lie on average at a potential that corresponds to half the voltage which is made available by the DC voltage source. At the end of service life, the second of the two reference points lies at a different potential, and the reference points are therefore at different potentials. The potential difference is used in order to charge a detection capacitor, the evaluation circuit advantageously being realized such that it is possible to set a voltage level upon the overshooting of which a deactivation of the half-bridge arrangement is effected.
The term xe2x80x9crelampingxe2x80x9d is known in conjunction with the replacement of a defective lamp. In the case of a lighting system comprising a plurality of lamps, this is understood as making possible the use of a new lamp without the need to switch off the supply voltage and therefore switch off the other lamps. However, the aim is to ensure that the line voltage connected during the entire replacement operation causes the newly inserted lamp to come on again immediately. Circuit structures are also known for this purpose. The disadvantage in the mode of procedure of the prior art resides in that the additional realization of the relamping function makes mass produced ballasts substantially more expensive, for which reason they are frequently omitted. The result is therefore expensive ballasts for which an end-of-life detection and relamping are realized, and there is a second category of ballasts, for which only the end-of-life detection is realized. In the case of the last mentioned ballasts, it is necessary, for example, for all the lamps to be switched off when, for example, replacing a fluorescent lamp in a factory hall, in order thereby to reset the end-of-life detection. Only after all the lamps have been switched off can a new lamp be inserted instead of the aging lamp. Subsequently, all the lamps can be switched on again. Such interruptions are undesirable, especially in large factory halls.
The object of the present invention therefore consists in making available a cost-effective realization of the end-of-life detection, and of the relamping function.
This object is achieved according to the invention by virtue of the fact that the generic protective circuit also has a third resistor that bridges the decoupling capacitor, and a fourth resistor that connects the first reference point to the positive pole of the DC voltage source, the first, the second, the third and the fourth resistor being selected such that the first and the second reference point are at the same potential without the fluorescent lamp inserted.
The invention is based on the idea of designing the end-of-life detection circuit or realizing the relamping function such that as many components as possible are used jointly. It is thereby possible in the case of a mass produced product such as the present protective circuit to realize the relamping additional function cost-effectively virtually without additional outlay, the result being a very desirable price reduction.
Here, the idea consists in that, with the fluorescent lamp removed, two inputs of the comparator, which detects asymmetry, are supplied with identical potentials which reset the switching-off of the half-bridge arrangement.
As already mentioned above, the two reference points lie on average at half the potential of the DC voltage made available by the DC voltage source. This is usually what is termed the DC link voltage, and is usually provided at a DC link capacitor. In a particularly preferred realization of the invention, the ratio of a first resistor to the fourth resistor is of the same magnitude as the ratio of the second resistor to the third resistor. Particularly in the case when the ratio is selected as 1, even with the fluorescent lamp removed, the two reference points lie at a potential that corresponds to half the DC voltage made available by the DC voltage source.
It is also preferred to use suitably dimensioned voltage dividers to apply only a lower voltage to the comparator. This results in a further cost reduction. For this purpose, the first resistor comprises a first and a second component resistor connected together in series, and the second resistor comprises a third and a fourth component resistor connected together in series, the first reference point being connected to the tie point of the first component resistor and the fourth resistor, and the second reference point being connected to the tie point of the third resistor and the third component resistor, and the first input of the comparator being connected to the tie point between the first and second component resistor, and the second input of the comparator being connected to the tie point between the third and the fourth component resistor. It is not necessary in this embodiment for all the resistors of the voltage dividers to be designed as high-voltage resistors. The comparator and evaluation circuits need likewise only be suitable for low voltage. However, it is sufficient to provide one high-voltage resistor per voltage divider, which results in a further cost reduction.
It is preferred for the ratio of the sum of the first and second component resistor to the fourth resistor to be equal to the ratio of the sum of the third and fourth component resistor to the third resistor. In the case in which the ratios are again selected as one, with the fluorescent lamp removed, the two reference points lie in turn at a potential that corresponds to half the DC voltage made available by the DC voltage source.
A particularly expedient realization of the comparator provides that the comparator comprises a first and a second switching element, which in each case comprise a working, a control and a reference electrode, the fourth component resistor comprising a fifth and a sixth component resistor connected together in series, the tie point between the first and the second component resistor being connected to the reference electrode of the first and to the control electrode of the second switching element, the tie point between the third component resistor and the fifth component resistor being connected to the control electrode of the first switching element, the tie point between the fifth and the sixth resistor being connected to the reference electrode of the second switching element, and the working electrode of the first switching element and the working electrode of the second switching element being interconnected and being connected to frame via a series circuit composed of a fifth resistor and the detection capacitor. In addition, the ratio of component resistors three, five and six can be used to set the potential difference that leads to starting the charging of the detection capacitor. The comparator is realized in a very simple and cost-effective form in this embodiment.
It is preferred for the ratio of the fourth resistor to the sum of the first and the second component resistor to be equal to the ratio of the third resistor to the sum of the third, fifth and sixth component resistor. Particularly in the case when the ratio is equal to 1, with the fluorescent lamp withdrawn the reference points are in turn at a potential that corresponds to half the DC voltage made available by the DC voltage source.
The evaluation circuit can comprise a holding element with a trigger potential and be designed in such a way that as soon as the trigger potential point has assumed a predetermined potential, in particular owing simply to a single pulse, the holding element can be activated in order to deactivate the half-bridge circuit until a resetting operation is triggered by removal of the fluorescent lamp. This measure ensures reliable deactivation of the half-bridge arrangement, and thus a particularly high reliability for the protective circuit according to the invention.
Between the comparator and the trigger potential point of the holding element can be arranged a first threshold component, in particular a Zener diode, with the aid of which it is possible to set the threshold upon the overshooting of which deactivation of the half-bridge circuit is triggered. This measure permits the holding element to be activated in the case of a prescribable voltage across the detection capacitor.
It is particularly advantageous for the combination of end-of-life detection and relamping function to be further combined with a starting-voltage-limiting circuit, for which purpose a starting-voltage-limiting circuit is connected to the trigger potential point in such a way that the same holding element can be activated upon detection of an overshooting of a predetermined starting voltage. Consequently, the holding element need be constructed only once, and this results in a further substantial cost reduction.
In this case, the starting-voltage-limiting circuit can have a measuring element for measuring a variable proportional to the starting current, such that the value of this variable can be used to activate the holding element. This embodiment utilizes the fact that the starting current is approximately proportional to the starting voltage and can therefore be used as a measure of the starting voltage. Since the starting current is easier to measure than the starting voltage, this results in a simpler design of the circuit arrangement.
It is preferred to arrange between a potential point of the starting voltage limiting circuit whose potential is proportional to the starting current, and the trigger potential point of the holding element a second threshold component, in particular a Zener diode, with the aid of which it is possible to set the threshold upon the overshooting of which a deactivation of the half-bridge circuit is triggered. This variant permits a particularly simple adaptation of the potentials of the starting-voltage-limiting circuit to the potentials of the holding element.
The measuring element can be, in particular, a resistor which is arranged in series with one of the half-bridge switches. This embodiment is based on the finding that the starting current is also made available by the half-bridge arrangement, and therefore the current flowing through the half-bridge arrangement is proportional to the starting current. A variable proportional to the starting current can be determined with particular ease by virtue of the fact that a resistor is arranged as measuring element in series with one of the half-bridge switches.
Switching a storage capacitor between positive and negative poles is generally customary. Operating circuits exist for fluorescent lamps in the case of which the voltage across the said storage capacitor rises with rising amplitude of the starting voltage. What are termed xe2x80x98pump circuitsxe2x80x99 are a type of operating circuit that have this property. In the case of these circuits, it is possible to monitor the starting voltage by monitoring the voltage across said storage capacitor. For this purpose, the trigger potential point of the holding element is connected via a starting-voltage-limiting circuit to the voltage of the storage capacitor. In the simplest case, the starting-voltage-limiting circuit consists of a resistor which adapts the voltage across the storage capacitor to the trigger voltage, required for triggering, at the trigger potential point.
It is preferred for the embodiments according to the invention also to comprise suitable filter circuits in order to provide DC voltages for evaluation at the reference and potential points. As is evident to the person skilled in the art, the half-bridge arrangement converts the DC voltage made available by the DC voltage source into an AC voltage that is mirrored in the downstream protective circuit. DC voltages are substantially of interest for evaluating the signals at the reference points, and so it is ensured by means of suitable filter circuits, for example using capacitors, that the same are provided for further processing.
Further advantageous embodiments are to be gathered from the subclaims.